Magnetic resonance imaging (MRI) and x-ray fluoroscopic imaging are important medical visualization tools used for image-guided interventional procedures. Each method provides its own advantages: MRI provides excellent soft tissue contrast, three-dimensional visualization, physiological information, and the ability to image in any scan plane, while x-ray imaging offers much higher spatial and temporal resolution in a projection format, useful for visualization and placement of guidewires, catheters, stents, and other medical devices. Combining the two imaging systems therefore offers significant benefits over using each system alone. Currently, several approaches are used for combining the systems. In one, an x-ray fluoroscope is located in a room adjacent to the MRI system. In another, the x-ray and MRI systems are in the same room, but the patient must be moved out of the magnetic field to be imaged by the x-ray system. Moving the patient is undesirable, because it is time consuming, possibly dangerous, and can render the images inconsistent. Therefore, one wants to minimize the distance between the two systems, and perhaps overlap them. This will place critical components of the x-ray system within a high magnetic field.
The ideal system is one in which x-ray imaging and magnetic resonance imaging can be performed in the same location, eliminating the need to move the patient. Before a combined MRI and x-ray system can be constructed, however, the individual systems must be modified to ensure that the high magnetic field of the MRI system does not affect the x-ray system, and that the x-ray system does not disturb the operation of the MRI system. For example, conventional x-ray fluoroscopy detectors are image intensifiers, which are exceedingly sensitive to magnetic fields and therefore cannot be used near, let alone inside, an MRI system. However, flat panel x-ray detectors that are relatively immune to magnetic field effects are now available.
A major obstacle to combining MRI and x-ray systems is the x-ray source, which consists of an x-ray tube and its housing. X-rays are generated using an x-ray tube, in which electrons are accelerated from a heated cathode to an anode by a very high potential (e.g., 150 kV). Interactions between the high energy electrons of the beam and atoms of the anode target material cause deceleration of the electrons and production of x-ray photons.
FIG. 1 is a schematic diagram of an x-ray tube 10 of the prior art. The tube 10 is evacuated and contains a tungsten filament cathode 12 and a more massive anode 14, typically a copper block 16 with a metal target 18 plated on or embedded in the copper surface. The target 18 is most often tungsten, but other metals can be used, such as chromium, copper, molybdenum, rhodium, silver, iron, or cobalt. Separate circuits are used to heat the filament 12 and to accelerate the electrons to the target 18. The accelerating potential determines the spectrum of wavelengths (or photon energies) of the emitted x-rays. A high voltage is connected between the cathode 12 and anode 18 to provide the accelerating potential. Typically, the anode and cathode voltages are plus and minus half of the accelerating voltage, respectively. X-rays generated at the target 18 exit the tube 10 through an x-ray transparent window 20 and are directed toward the object being imaged.
When an x-ray tube is operated within or near an MRI system, it experiences the static magnetic field Bo, as illustrated schematically in FIG. 2. The magnetic field at the location of the x-ray tube exerts a force on the electrons and may deflect or defocus the electron beam. The force on an electron is proportional to the cross-product of the velocity of the electron and the magnetic field; that is, only the velocity component that is perpendicular to the magnetic field is perturbed. This will alter the direction of the electron motion, thereby making the direction of the deflecting force time-dependent. In the example of FIG. 2, the macroscopic result of the time-dependent force is to produce an electron beam in the direction of B0, with an additional deflection of the beam v⊥drift in a direction perpendicular to both B0 and the electric field E. Because the ideal electron velocity is in the direction of the target, as is the acceleration caused by the electric field, unless the magnetic field is parallel to the electron beam, it always deflects the electrons away from the center of the target, possibly causing them to miss the target entirely. Thus the effect of the static magnetic field of the MRI system on the x-ray tube can be highly undesirable and may damage the tube if it is operated under non-ideal conditions, or it may lower the x-ray intensity to a level that is unacceptable. In the combined system, it is not desirable—indeed it may be impossible—to turn off the static magnetic field before acquiring x-ray images, and so the effect of the magnetic field on the x-ray tube must be addressed.
A number of combined magnetic resonance imaging and x-ray imaging systems are disclosed in the prior art. U.S. Pat. No. 5,713,357, issued to Meulenbrugge et al., discloses a combined system that minimizes or eliminates the distance an object being imaged must be displaced between the individual systems. In one embodiment, the object is displaced a small distance along a track between adjacent MRI and x-ray imaging systems with non-coincident fields of view. In another embodiment, the object is not moved and the fields of view of the two systems are coincident, but the x-ray imaging system is moved out of the MRI field of view during MR image acquisition. During x-ray imaging, the x-ray source is either out of range of the static magnetic field, passively shielded from the magnetic field, or positioned so that the electron beam is parallel to the magnetic field. In this alignment, the electron beam should not be deflected by the magnetic field.
U.S. Pat. No. 5,818,901, issued to Shulz, discloses a combined system with simultaneous MR and x-ray imaging and coincident fields of view. A solid state x-ray detector containing amorphous hydrated silicon, which is not affected by the magnetic field, is used in place of an image intensifier. The x-ray source is positioned far enough from the MR apparatus that the influence of the magnetic field on the x-ray source is slight. Additionally, the influence is reduced further by surrounding the source with a cladding material that shields the source from the magnetic field. The goal of the cladding or shielding is to reduce the magnetic field at the location of the x-ray source to a level where it can be tolerated.
U.S. Pat. No. 6,031,888, issued to Ivan et al., discloses an x-ray fluoroscopy assist feature for a diagnostic imaging device such as MRI or computerized tomography (CT). X-rays are generated using a rotating anode x-ray tube. There is no mention of the effects of the magnetic field on the x-ray source or of any methods to eliminate such effects.
A medical imaging apparatus containing both x-ray radiographic means and MRI means is disclosed in U.S. Pat. No. 6,101,239, issued to Kawasaki et al. The x-ray and MRI systems have coincident fields of view, and the timing of the image acquisition is controlled so that the x-ray pulses occur only when the gradient magnetic fields and RF magnetic fields fields of the MRI system are off. There is no mention of minimizing or eliminating the effect of the static magnetic field on the x-ray source.
These prior art references offer two solutions to the problem of electron beam deflection in the x-ray tube by the static magnetic field of the MRI system: shielding the tube or aligning the electron beam with the magnetic field. Sufficient cladding to completely eliminate the effect of the magnetic field on the electron beam may not be feasible. Aligning the tube with the field also has potential problems. First, the alignment may be very critical, i.e., have a very small tolerance, making it difficult to attain. Second, x-ray tube inserts typically have components that distort the magnetic field and pose additional difficulties that cannot be solved simply by aligning the electron beam with the magnetic field. The alignment also constrains the image plane, potentially to undesired orientations. Thus it would be advantageous to provide a more robust method for eliminating the effect of the static magnetic field of the MRI system on the electron beam of the x-ray tube.